As a transformer (voltage transformer), there is well known an electromagnetic transformer comprising windings wound around an iron core. The electromagnetic transformer is unsuitable for use in a power supply for a small-sized electric apparatus because it is bulky in size, is large in power consumption, and generates electromagnetic noise and heat. For example, for use in a high-voltage power supply in an electrostatic generating device or a back-lighting lamp of a liquid-crystal display, the transformer does not require a large output current but requires an output voltage between 1 kV and about several watts. In addition, it is required to reduce the electromagnetic noise, the power consumption, and the size.
On the other hand, since a piezoelectric transformer utilizing a piezoelectric phenomenon generates little electromagnetic noise and can be reduced in size, practical use is considered as a power supply transformer for a small-sized apparatus.
Referring to FIGS. 1(a) and (b), a conventional piezoelectric transformer 11 comprises a piezoelectric-ceramics rectangular plate 13, two surface electrodes 15 and 15 formed on the piezoelectric-ceramics rectangular plate 13 opposite to each other in a thickness direction at a part (hereinafter referred to as a first part) extending from one end to an approximate half in a longitudinal direction, and a plurality of internal electrodes 16 and 17 formed in the interior of the above-mentioned first part between the both surface electrodes with a space kept from one another in the thickness direction. Side electrodes 18 and 19 formed on confronting side surfaces of the above-mentioned first part, respectively, are connected to the surface electrodes 15 and 15, respectively, and to the alternate internal electrodes 16 and the remaining internal electrodes 17, respectively. Moreover, an end electrode 20 for output extraction is formed on the piezoelectric-ceramics rectangular plate 13 over an end surface of a half part (hereinafter referred to as a second part) opposite to the above-mentioned first part.
The above-mentioned first part of the piezo-electric-ceramics rectangular plate 13 is polarized by applying a DC voltage between the side electrodes 18 and 19. Specifically, the piezoelectric-ceramics rectangular plate 13 is polarized between adjacent electrodes of the surface electrodes 15 and 15 and the internal electrodes 16 and 17. The polarization directions are opposite to each other at both sides of each of the internal electrodes 16 and 17, as depicted by small arrows in FIG. 2(b). Furthermore, by applying a DC voltage between the both surface electrodes 15 and the end electrode 20, the second part of the piezoelectric-ceramics rectangular plate 13 is polarized in the longitudinal direction, as depicted by a large arrow in FIG. 1(b).
The piezoelectric transformer of the one-wavelength resonance type having the above-mentioned electrode structure is referred to as a Lozen type piezoelectric transformer.
The above-mentioned type including the plurality of internal electrodes as input electrodes will be referred to as a stacked type because it is actually formed by alternately stacking the internal electrodes and piezoelectric members during manufacture. On the other hand, another type is also known in which the polarization in the thickness direction is only one direction between the confronting surface electrodes 15 and 15 without any internal electrodes formed. This type will be referred to as a single plate type because no stacking is required during manufacture and it is implemented by a single piezoelectric member with electrodes formed on its surface.
Description will now be made as regards an operation of the Lozen type piezoelectric transformer illustrated in FIGS. 1(a) and (b).
Now, one of the side electrodes 18 and 19 is used as a ground terminal and the other is applied as an input voltage with an AC voltage having a frequency equal to a resonant frequency of the piezoelectric-ceramics rectangular plate 13 in a one-wavelength resonance mode of a longitudinal vibration. Then, the stacked-type piezoelectric transformer acts as a piezoelectric vibrator to vibrate with a displacement distribution and a strain distribution illustrated in FIGS. 2(a) and (b), respectively. At this time, an AC voltage is produced between each of the surface and the internal electrodes 15, 16 and 17 and the end electrode 20 due to the piezoelectric effect. The level of the voltage thus produced is generally determined by distances between the surface electrodes 15 and the internal electrodes 16 and 17, a distance between the surface electrodes 15 and the end electrode 20, and the input voltage. Specifically, in the piezoelectric transformer, a transformed voltage can be obtained by energy conversion utilizing the piezoelectric effect, that is, electric-mechanical-electric conversion.
In the meanwhile, with the recent demands for reduction in size and weight of electronic apparatuses, the piezoelectric transformer of this type is also required to be small in size and weight. For example, as for the piezoelectric vibrator having the dimension of the length (mm).times.the width (mm).times.the thickness (mm) 42.times.12.times.1.5, it is required that the width is reduced to half (42.times.6.times.1.5) to decrease its volume to half.
However, when the volume of the piezoelectric vibrator is reduced, transmitted electric power is decreased in proportion to the reduction in volume. In order to obtain a predetermined output, the vibration rate of the piezoelectric vibrator must be increased. Specifically, once the vibration mode and the length of the piezoelectric vibrator are determined, the vibration frequency is also determined. In order to increase the vibration rate in case where the width of piezoelectric vibrator is reduced to half as described above, an input voltage must be increased to increase the displacement (amplitude) of the piezoelectric vibrator.
However, the increase of the vibration rate in the piezoelectric vibrator brings about the increase of heat generation to result in the decrease in efficiency. Specifically, in case where a desired output can not be obtained at the vibration rate as high as a vibration level limit (this means the vibration rate at which the temperature (.DELTA.T) of the vibrator reaches a predetermined level due to heat generation at a high vibration rate and a large amplitude; the predetermined level can be selected, for example, as .DELTA.T=25.degree. C.) inherent to the piezoelectric ceramics, the heat generation exponentially increases and the efficiency is considerably decreased even if the vibration rate is increased over the vibration level limit. Accordingly, the desired output can not be obtained. Thus, in the conventional piezoelectric transformer having the Lozen type electrode structure, it is difficult to reduce the size.
It is an object of this invention to provide a piezoelectric transformer which is smaller in size than the conventional piezoelectric transformer but is still capable of obtaining an output equivalent to that of the conventional piezoelectric transformer at a vibration rate lower than a vibration level limit inherent to a material.